My favourite is to see what clothes I can get away with wearing for the interview. If it's during a normal work week, I'll suggest that if I go into my normal job wearing anything but t-shirt and jeans, they'll smell a rat. Most companies that I'm going to be a fit for really don't care what I wear, even for the interview, and will say so.
Of course I don't know what culture you're looking for. You might want to dress up super-smart because that's the environment you want to work in. My point is that dress-code is a strong indicator of culture.
I'm always interested in looking around the office, while people are still working. Get a sense of the vibe. If the people I'm meeting try to hide me from the office, I'll ask if I can take a look around. I want to feel the environment and decide if that's a place I want to spend 8+ hours a day.
Another big clue is the people who interview you. In my opinion, HR should never interview people for skilled roles, at least beyond a simple phone screen. Even if it's just a manager, on his own, that's a red-flag. The kind of place I want to work let's a team decide who they want to work with.
Again, you might disagree with me there, or you might not be looking for a skilled role, but it's always a good way of looking at the culture of a company.
Also, just be yourself. Let them decide if you're going to be a fit or not. There's nothing more dangerous in an interview than pretending to be what you think they want. If you're right, you'll never be able to sustain the illusion; if you're wrong, you might scare off a good match.
All this said, there is no substitute for asking the right questions. None.